ABSTRACT In our normal busy lives we rarely hear isolated sounds. In most situations, such as a cocktail party, many sound sources are present and their waveforms simply add together at the listener's ears; all that listeners hear is a single complex waveform. Remarkably, listeners are able to break down this complex waveform, segregate the components by frequency and other cues, and group them together to form a coherent percept of the separate sources; this process is commonly referred to as auditory scene analysis. Hearing aid and cochlear implant users have particular difficulty with this process and many avoid environments where multiple sound sources are present. A better understanding of the mechanisms underlying auditory scene analysis may solve some of these difficulties. Auditory scene analysis is a process that utilizes the features of the sound (bottom-up processing) and the expectations of the listener (top-down processing). The long-term objective of the proposed research is to understand the mechanisms of the bottom-up processing and how it interacts with the top-down processing. The proposed research aims to understand the physiology of the binaural mechanisms used to extract a single sound from an environment with multiple sound sources. The auditory system can compare and contrast the input of the two ears and use these binaural cues to segregate sounds. Little is understood about the physiological processes that compare the input of the two ears when multiple sounds are present. This lack of knowledge has become a barrier to further improving the hearing of people with hearing aids or cochlear implants in both ears. One aim of the proposed research is to determine the binaural mechanisms used in extracting a single sound when multiple sound sources are present. The present proposal will use multiple complex sounds (harmonic complexes) to identify the binaural neural mechanisms used to segregate sounds. The second aim of the proposed research is to determine how top-down processes affect the binaural mechanisms used to segregate sounds. The proposed research will combine electrophysiology in the inferior colliculus with auditory cortical inactivation techniques to determine the interaction between binaural mechanisms and top-down processing. The results will provide a framework for binaural advantages in complex environments that may help with maximizing the benefit from bilateral hearing aids and bilateral cochlear implants.